Method for skinning the seeds of legumes, drupes, and grains

ABSTRACT

A method and apparatus for removing the skins or hulls from seeds, including those of legumes, grains, drupes, silques, and achenes involves wetting the seeds with an alkaline solution and then with a peroxygen solution. The two substances react underneath the skin to liberate gas between the seed meat and the skin. The gas bubbles blister the skin, causing it to become loosened. Slight mechanical action is used to dislodge the blistered skins from the seed meats.

This application is a 371 of PCT/US95/02176 filed Feb. 21, 1995 aContinuation-in-Part of now-allowed application Ser. No. 08/035,721,filed Mar. 23, 1993 now U.S. Pat. No. 5,391,389.

FIELD OF THE INVENTION

The present invention relates to the field of removing the skin or hullsfrom seeds used for food consumption, such as nuts and the seeds oflegumes, grains, drupes, silques, achenes, and other products.

BACKGROUND OF THE INVENTION

The seeds of many fruits such as legumes, grains, silques and drupes arecomprised of a meat that is covered by a skin or seed coat (the term"seed" as used herein will be intended to include the seeds of productssuch as legumes, grains, drupes, achenes, silques and other types ofseeds used for food products). Beans, corn, and coffee beans areexamples of seeds from legumes, grains, and drupes, respectively. Rapeseed is one example of seed which comes from silques. Sunflower seedsand buckwheat are seeds which come from a type of fruit called anachene.

A nut is a one-seeded fruit which, for purposes of this application willbe referred to as a "seed". The nut kernel, which is the component ofthe nut that remains after shelling, is comprised of a nut meat that iscovered by a brownish skin. Skinning of a nut kernel is commonlyreferred to as "blanching."

Removal of the skin or hull for such seeds is desired for a variety ofreasons. For example, some skins, such as those found on coffee beans,hazelnuts, and Brazil nuts have unpleasant bitter tastes. For otherseeds, such as almonds, removal of the skin is desired to prevent theskin from coloring foods that are prepared using the seeds. Skin or hullremoval for seeds in the grain family, such as wheat and corn, isdesired for the production of certain grain products such as wheat flourand masa. It is also desirable to remove the skin or hull from certainseeds so that the skin or hull may be used as dietary fiber for humansor other animals.

Various processes have been used for removing the skin from seeds. Theseprocesses have enjoyed limited success. For the most part, existingprocesses are unsatisfactory because they do not completely remove theskin without abrading away significant amounts of meat or because theyinvolve temperatures or substances which adversely affect the taste,appearance or shelf life of the seeds.

Because many seeds are small in size, it is difficult to abrade theskins from the seeds without significant loss of meat. Mechanicalabrasion of these seeds may result in loss of a substantial portion ofthe valuable seed meat. Certain nuts, such as hazelnuts and Brazil nuts,have a tightly adhering skin which is difficult to remove due to deeplyembedded vein networks. The convoluted surfaces of these and other nuts,such as almonds, make it difficult to abrade the skins from the nutswithout significant loss of nut meat. Often mechanical abrasion of thesenuts results in a loss of 10 to 20% of the valuable nut meat.

Some existing processes for skinning seeds involve removing the skins byhigh pressure jets of water or by scalding water. Unfortunately, highwater pressure also may erode areas of the meat and create pits in thesurface of the seeds. Breakage is also known to occur, which isparticularly undesirable for products which are marketed in whole form.Further, seeds treated with these methods tend to absorb water and thusrequire drying, a process which can weaken the flavor of the seeds.

The skins of corn and beans such as red beans, broad beans and peas areoften removed after the products have been boiled, because boilingloosens the skins of these products. This process causes the beans toabsorb water and thus has limited applicability in contexts where adryer product is desired. The hulls of coffee beans are commonly removedusing pneumatic separators, which are sometimes unsuccessful atcompletely removing the hulls, or by soaking which requires substantialdrying time following soaking.

In a common chemical process for removing skins from some seeds such asnuts, seeds are first immersed in an alkaline bath, subsequentlyimmersed in an acid bath, and finally subjected to mechanical abrasion.However, the acid leaves the seeds with an unpleasant bitter taste andmay reduce their shelf life. Moreover, the concentrations of thechemical solutions dilute over the soaking period because of debrisintroduced into the solutions by the seeds. Dilution makes regulation ofthe concentration of the solutions highly difficult and renders theseprocesses unfeasible for large scale blanching operations.

Some products are skinned using hot caustic soaking. In hot causticsoaking, the products are soaked in hot (i.e. approximately 180° F.)sodium hydroxide solution and subsequently peeled using mechanicalabrasion. During soaking the hot caustic penetrates below the skin,softening the flesh underlying the skin. This softened flesh is abradedaway during the mechanical peeling step, resulting in loss of valuableproduct. Moreover, use of sodium hydroxide--a caustic agent frequentlyreferred to as lye--produces toxic effluents which, for environmentalreasons, are difficult to dispose of. Processes using lye are stronglyalkaline and produce unstable alkaline solutions above pH 9 whichrequire neutralization before final disposal.

Finally some seed products, such as hazelnuts, are skinned by roastingfollowed by light mechanical abrasion of the skins from the seeds.Although this method is satisfactory at removing the skins, it producesseeds having a roasted flavor.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method andapparatus for removing the skin from seeds without adversely affectingthe taste, appearance, or shelf life of these products and withoutabrading away valuable meat. It is another object of the presentinvention to provide a method for quickly removing the skin from seedswithout producing toxic emissions, effluents, or residues.

The present invention involves wetting the seeds in two steps: firstwith an alkaline solution and then with a peroxygen solution. The twosubstances then react to liberate gases underneath the skin. The gasbubbles blister the skin or hull, causing it to become loosened. Theperoxygen solution also prevents the discoloration of the seeds thatwould otherwise be caused by the alkaline solution.

A mechanical device is used to separate the blistered skins from themeats.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a skinning apparatus usable toremove the skins of the seeds of legumes, grains, drupes and the likeaccording to the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of this discussion, the term "skin" will be used torepresent both seeds and hulls. The term "seed" will be used torepresent a seed after it has been removed from a shell, if the seed isone (e.g. an almond or hazelnut) which comes in a shell, or after it hasbeen removed the fruit, pod, cone or other structure in which it grows.

The method of the present invention generally involves wetting of theseeds with an alkaline solution, wetting of the seeds with a peroxygensolution, and mechanical removal of the skins from the seeds. A methodof this type as applied to nuts is described and claimed in U.S. Pat.No. 5,391,389, the application of which the present application is acontinuation-in-part and which is incorporated herein by reference.

The first step of the process preferably involves wetting the seeds withan alkaline solution. Alkaline solutions are known to partially breakthe bonds which adhere the cellulosic skin to the seed meat. Thepreferred method utilizes food grade sodium carbonate, although otheralkaline agents such as sodium bicarbonate, sodium silicates, sodiumphosphates, and borates have been shown to work well. Alkaline salts oflithium, potassium, magnesium, and calcium will work satisfactorily butare more costly to use.

It is believed that alkaline solutions of 0.1% to saturated (e.g.0.1%-10% sodium bicarbonate or 0.1%-15% sodium carbonate) havingapproximate temperatures of 40 to 200° F. are useful in carrying out thefirst step of the process, particularly when applied to the seeds fordurations of between 5 seconds and 300 minutes.

The preferred concentration for the alkaline material is in the range ofapproximately 1-3%. The preferred temperature range is approximately80°-150° F. so that the seeds may be quickly skinned without becomingcooked or soggy. For the same reason, the preferred application time forthe alkaline step is approximately 30 sec-10 minutes.

Removal of the skins from nuts (a process often called "blanching"),alkaline solutions of 1% to saturated having approximate temperatures of5 to 49° C. (40 to 120° F.) are preferable for use in the process,particularly when applied to the nuts for durations of between 20seconds and 20 minutes. In the preferred method for blanching nuts, thenuts are wetted with 10% Na₂ CO₃ at 35° C. (95° F.) for 2 to 3 minutes.Exposure time may be increased to up to 10 minutes if accompanied by areduction in temperature to 21° C. (70° F.). While the blanching will besuccessful at prolonged exposure times and/or temperatures in excess of50° C. (122° F.), these conditions tend to make the nuts more fragileand also may reduce shelf life and for these reasons should preferablybe avoided.

The seeds are next preferably allowed to drain at ambient temperature.While draining is not required for successful skinning, it allows thealkaline material to soak into the skin in order to thoroughly moistenthe skin. This is important because it is preferable for the reactionbetween the alkaline solution and the peroxygen solution applied in thesecond step to occur between the skin and the nut meat. For some seeds,draining at ambient temperature may also be beneficial in that it canallow cooling of the seeds and it thereby reduces the likelihood ofrancidity, breakage, and reduced shelf life. Drying at ambienttemperature may last up to 30 minutes without significant adverseconsequences.

After draining, the seeds are wetted using a peroxygen compound.Hydrogen peroxide is preferred for economic reasons and also because itremoves the discoloration of the meats caused by the alkaline solution.Other preferred solutions are sodium peroxides, percarbonates,perborates and persulfates, however these are more expensive chemicalsand are thus more costly to use in the process.

It is believed that peroxygen concentrations of 2-35% (and preferably8-25%) work particularly well in the process when applied to the seedsat temperatures of approximately 8° to 180° F. for durations of between1/2 minute and 300 minutes. The preferred ranges for application timeand temperature for the peroxygen step are approximately 30 seconds-10minutes and 120°-160° F.

For nuts, peroxygen concentrations of 10-40% work particularly well inthe process when applied to the nuts at temperatures of approximately10° to 79° C. (50 to 175° F.) for durations of between 1/2 minute and 20minutes. Although blanching will be successful when the nuts are treatedwith peroxygen solutions having temperatures greater than 80° C. (176°F.) or if they are exposed for periods of longer than 200 seconds toperoxygen solutions having temperatures of 77° C. (170 F.) or higher,these conditions should preferably be avoided to prevent cooking,rancidity, and breakage of the nuts. For economic reasons, the nuts arepreferably treated for 150 seconds with a 15% solution of hydrogenperoxide maintained at 65° C. (149° F.).

Once the seeds have been exposed to the hydrogen peroxide, the hydrogenperoxide and the sodium carbonate chemically react to liberate gas.Because the alkaline material has been allowed to permeate the skin, gasbubbles form between the seed skin and the seed meat causing blisteringand loosening of the skin. An appropriate mechanical method such aslight brushing, rubbing or tumbling is used to dislodge the loosenedskins and the seeds are allowed to dry.

EXAMPLES

Following is experimental data obtained during application of theprocess to legume seeds (specifically, beans), grain seeds(specifically, corn) and drupe seeds (specifically, coffee beans). Thisdata is given by way of example, and should not be construed as limitingthe claims herein.

A. Beans

The process according to the present invention was applied to 50 g ofbeans. Pale pink kidney beans were used in the experiment, although theprocess is believed useful for many types of beans, including lima,pinto, garbanzo, and black beans, particularly when the beans are fresh.Fifty grams of beans were wetted with a 2% solution of sodium carbonatehydrate at 125° F. for approximately 3 minutes to thoroughly moisten theskin. The quantity of solution used was 10 g, an amount just sufficientto completely moisten the skin with only a small amount of excess.

The beans were drained and subsequently wetted with approximately 10 mlof 20% hydrogen peroxide solution at 155° F. for approximately 3minutes. The amount of hydrogen peroxide solution applied to the skin isan amount sufficient to wet the skins with only a small amount of excesssolution. By the end of the three minutes, the beans skins were puffedand separated from the bean kernel. The skins could be easily removed bymild mechanical action, such as brushing, rubbing, and/or tumbling.

Although the present example has been given with respect to beans, itshould be understood the process according to the present invention maysuccessfully remove the skins from the seeds of various other legumes,including, but not limited to, pinto beans, garbanzo beans, black beans,lima beans, and peas.

B. Corn

Three experiments were carried out in which the process according to thepresent invention was used on corn. In the first run, fresh corn kernelswere wetted for 2 minutes using an 8% solution of NaHCO₃ at 140° F., andsubsequently allowed to drain for 2 minutes at ambient temperature.Next, a 15% solution of H₂ O₂ at 160° F. was used to wet the kernels for2 minutes. After the hydrogen peroxide step, the corn skin was loosenedand mechanically removable, but with slight difficulty. A third step ofsoaking the kernels in 100° F. water facilitated skin removal.

In the second run, fresh corn kernels were first wetted for 2 minutesusing an 8% solution of NaHCO₃ at 180° F., and subsequently allowed todrain for 2 minutes at ambient temperature. A 15% solution of H₂ O₂ at140° F. was then used to wet the kernels for 2 minutes. After thehydrogen peroxide step, the corn skin was substantially loosened andeasily removable by light mechanical action or by plucking the skin fromthe kernel using the fingers.

Finally, in the third run kernels were wetted using an 8% solution ofNaHCO₃ at 180° F. for 1 minute and 40 seconds and allowed to drain atambient temperature for 2 minutes. For the peroxygen step, the kernelswere wetted using a 10% solution of H₂ O₂ at 140° F. for 1 minute and 40seconds. Again the corn skin was substantially loosened and easilyremovable.

Corn is but one example of a grain seed for which the process of thepresent invention may be applied. The process is believed applicable forremoving the hulls from many other grains, including wheat, rice, oatsand barley.

C. Coffee Beans

Three experiments were also carried out in which the process accordingto the present invention was used on raw coffee beans. As with the beansand corn, the coffee beans were wetted, not soaked, with the alkalineand peroxygen solutions. The conditions and results are as follows:

First Run

Step 1--8% NaHCO₃ at 180° F. for 20 seconds, followed by 1 minute draintime at ambient temperature

Step 2--10% H₂ O₂ at 140° F. for 20 seconds.

Results--entire skin easily removed.

Second Run

Step 1--8% NaHCO₃ at 180° F. for 15 seconds, followed by 1 minute draintime at ambient temperature

Step 2--10% H₂ O₂ at 140° F. for 15 seconds.

Results--entire skin easily removed.

Third Run

Step 1--8% NaHCO₃ at 180° F. for 15 seconds, followed by 1 minute draintime at ambient temperature

Step 2--10% H₂ O₂ at 120° F. for 15 seconds.

Results--entire skin easily removed.

For purposes of comparison, raw coffee beans were soaked in 180° waterfor 2 minutes. Only portions of the skins could be removed withoutsubstantial difficulty.

It should be appreciated that the process of the invention may beapplied to drupe seeds other than coffee beans. For example, shelledalmonds and walnuts are seeds of drupes to which the process of thepresent invention is known to be applicable. During experiments applyingthe process of the present invention to shelled almonds follows, thenuts were first wetted using a 4% solution of NaHCO₃ at 160° for 30 sec,and subsequently wetted with a 15% H₂ O₂ at 160° for 60 sec. The skinsof the almonds were easily removable by light mechanical means.

In addition to coffee beans, almonds, and walnuts, the process of thepresent invention is also believed applicable to pecans, cashews, andpistachios. The process is also believed useful for removing the skinsfrom the seeds of other drupes, such as peaches and apricots.

D. Nuts

In another experiment, the process according to present invention wasapplied to hazelnuts under the following conditions:

Step 1: 4% NaHCO₃ at 160° for 30 sec, followed by draining

Step 2: 15% H₂ O₂ at 160° for 90 sec

Results: Skins removable.

Although the experiment was performed using hazelnuts, it should beappreciated that the process is applicable to other forms of nuts,including Brazil nuts, hickory nuts, and chestnuts.

A diagrammatic flowchart illustrating an apparatus which may be used forcarrying out the invention is shown in FIG. 1. The apparatus generallycomprises a pair of rotating spray drums 10, 12 which apply the sodiumcarbonate and hydrogen peroxide solutions to the seeds, a skindislodging device 14 which mechanically removes the skins at the end ofthe process, and an air separator (not shown) which separates theremoved skins from the seeds. Because, as described above, thetemperature, concentration, and duration of exposure must be maintainedwithin specified ranges, the apparatus is preferably designed to ensurethat the solution applied to the seeds is maintained at thepredetermined temperature and concentration values.

The seeds are carried on a standard conveyer 18 into the first spraydrum 10. The spray drum 10 is positioned at a slight downward angle andincludes a standard screw auger (not shown) for pushing the seedsthrough the spray drum. The various parameters for the spray drumconfiguration are dependent upon the amount of exposure time which isdesired for the seeds. Positioned inside the spray drum 10 are twonon-rotating rows 20, 22 of spray nozzles 24 which spray sodiumcarbonate solution onto the seeds as the seeds are rotated inside thespray drum. Two variables, the speed of rotation of the spray drum andthe area of the inner surface 26 of the spray drum falling within thepath of the spray 27, determine how long the seeds are exposed to thesolution per rotation of the drum. A second two variables, the angle ofthe spray drum and the speed at which the screw auger pushes the seedsout of the drum, determine how long the seeds are inside the drum andthus how many times the seeds pass through the spray 27. These fourvariables must therefore be chosen based upon the length of time forwhich it is desired to expose the seeds to the solution. A variablespeed spray drum is preferred to enable exposure time to be adjusted.

The spray nozzles 24 are preferably arranged in two parallel rows 20, 22within the spray drum 10. The first row 20 is connected to a freshsource of sodium carbonate (not shown) while the second row 22 receivessodium carbonate from a recycle tank 28 positioned at the exit 40 of thespray drum. It is not necessary for the spray nozzles to deliver a highpressure stream of solution onto the seeds because the fluid pressure isnot intended to be used for mechanical removal of skin. Because recycledsolutions are delivered through the nozzles on the second row, largenozzles are preferable to prevent debris from clogging them. This willnaturally result in a fairly low pressure spray.

The apparatus is preferably configured to recycle the runoff sodiumcarbonate, which is the sodium carbonate that runs off of the seedsduring and after spraying,back onto the seeds through the second row 22of spray nozzles. The runoff solution may carry dirt and other materialsoff the seeds and thus may become diluted. It is important, however, tomaintain the concentration of the solutions being used to treat theseeds at a predetermined level so that the preselectedconcentration-dependant values for temperature and exposure time will beadequate for successful blanching of the seeds. As will next bedescribed, the preferred apparatus recycles the runoff solution whilemaintaining the overall concentration of sodium carbonate being appliedto the seeds.

A recycle tank 28 located at the bottom of the spray drum 10 collectsthe runoff solution as it flows out of the spray drum. This collectedsolution is passed through a trash screen 30, and a pump 31 pumps it tothe second row 22 of spray nozzles 24 where it is sprayed onto theseeds. Positioned in the recycle tank 28 is a monitor 32 which detectsthe concentration of the runoff solution and which signals aconcentration regulator 34 if adjustments in concentration arenecessary. The concentration regulator 34 is located at the source ofthe fresh solution feeding into the first row 20 of spray nozzles 24. Ifthe monitor 32 detects that the concentration of the solution in therecycle tank 28 is less than the predetermined concentration fortreating the seeds, the concentration of the incoming fresh solution isincreased so that the concentration of the mixture of fresh and recycledsolutions reaching the seeds is maintained at the predetermined level.

The temperature of the solution should also be maintained atpredetermined levels. The fresh source of sodium carbonate is heated tothe desired temperature by conventional heating means (not shown). Athermostatic regulator (not shown) positioned in the recycle tankmonitors the temperature of the run-off solution and makes necessaryadjustments to the temperature of the runoff solution.

A conveyer 36 for transporting the seeds to a holding hopper 38 ispositioned near the exit 40 of the spray drum 10. The holding hopperholds the seeds during the draining and drying period. After thedraining and drying period, an elevator 42 carries the seeds from theholding hopper to a feed conveyer 44 which transports them into thesecond spray drum 12 for application of hydrogen peroxide.

The second spray drum 12 operates the same as the first, withconcentration and temperature being monitored and adjusted as describedabove.

A conveyor 58 carries the seeds from the second drum 12 to a skindislodging device 14, such as a rotating cylindrical brush, whichseparates the skins that have been loosened by the reaction of thesodium carbonate with the hydrogen peroxide as described above. Acylindrical brush may be preferable because it does not abrade the seedsbut rather dislodges the already loosened skins by poking through themand pulling them away from the seeds. However, other forms of dislodgingdevices (including pneumatic separators, rubbing and/or tumblingdevices) may be more appropriate depending on the size and fragility ofthe seeds being processed.

A final conveyor 16 next carries the seeds from the cylindrical brush orother skin dislodging device 14 to an air separator (not shown). At thispoint the seeds are likely to be mixed together with the pieces of skinthat were removed by the dislodging device. The air separator uses anair stream to separate the relatively heavy seeds from the lighterweight skins.

While certain details of the invention have been illustrated anddescribed herein it should be obvious that many modifications thereofmay be made which fall within the scope of the following claims. Forexample, while specific examples having been given in which the processof the present invention is applied to the seeds of specific types offruit (grains, drupes, and legumes) it should be understood that theprocess may be applied to other seeds, including those (such as pinenuts, rape seeds, sunflower seeds, safflower seeds, sesame seeds, andmany others) for which experimental results have not been given.

We claim:
 1. A method for removing skin from seeds in the class consisting of beans, corn, coffee beans and grains, the method comprising the steps of:(a) providing a first solution of an alkaline compound, the first solution having a concentration of approximately 0.1% to saturated and a second solution of a peroxygen compound that is capable of reacting with the first chemical solution to form a gas, the second chemical solution having a concentration of approximately 2-40%; (b) wetting a seed in the class consisting of beans, corn, coffee beans, and grains with the first solution; (c) wetting the seed with the second solution, forming a gas between the seed and a skin on the seed to loosen the skin; and (d) removing the skin from the seed.
 2. The method of claim 1 wherein step (a) provides sodium carbonate solution as the first solution and hydrogen peroxide solution as the second solution. 